Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output and a flow of exhaust gases. The exhaust gases produced by the engine can contain a complex mixture of air pollutants generated as byproducts of the combustion process. For example, the exhaust gases can include a high concentration of NOx when the combustion process generates temperatures greater than about 1500° F.
Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine. For this reason, engine manufacturers have implemented a variety of different methods for reducing the type and/or amount of pollutants generated by the engine. One method used by some manufacturers includes reducing combustion temperatures of the engine below the threshold temperature at which NOx formation occurs.
An exemplary NOx-reducing system is disclosed in U.S. Pat. No. 7,028,648 that issued to Hasegawa et al. on Apr. 18, 2006 (“the '648 patent”). In particular, the '648 patent discloses a system for an engine having a plurality of cylinders sharing a single crankshaft. Combustion characteristics of the cylinders are improved by taking out gas from an expanding cylinder and directing the gas into a compressing cylinder. This flow of gas between cylinders is facilitated by way of a dedicated conduit and cam-operated valves associated with each cylinder. By taking out gas from the expanding cylinder at a time of highest temperature, the overall temperature of that cylinder is reduced, thereby also reducing an amount of NOx formed within the cylinder. In addition, by directing the removed gases into the compressing cylinder, a greater power output can be subsequently generated by the compressing cylinder.
Although the system in the '648 patent may help to lower NOx production, it may also be problematic. In particular, the gases being transferred between cylinders include burned or partially burned molecules (soot and particulate matter), which can clog the conduit and/or associated valves. In addition, the transferred gases are at an elevated temperature, which could result in excessive NOx production by the cylinder receiving the gases. Further, it may be difficult to precisely time opening of the gas-transferring valves at peak combustion such that pressures and/or temperatures in the donating cylinder are maintained at desired levels.
The disclosed engine system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.